In connection with microwave communication systems, such as those used in communications satellites, networks are provided for generating antenna beam signals which are used to drive transmit arrays which in turn form transmit beams to transmit communication signals to an intended destination. Early types of beam-forming networks were used in frequency scanning radar to form a frequency scanning beam. This early type of beam-forming network comprised a single periodic delay line in the form of a meandering transmission line and only a single beam was formed at a time. A relatively large scan angle was covered by the beam in a series of small angular steps, each step corresponding to a frequency step in the radar transmitter. Such a system is a time sequential arrangement.
In other applications, resonant circuit delay networks were employed to achieve frequency addressability of antenna beams Resonant circuits were employed because the frequencies and bandwidths involved were relatively low and transmission lines having the required delays would have been too long to be practical.
The above-discussed beam-forming networks are unsuitable for high frequency communication satellites wherein it is desirable a simultaneously form a plurality of steerable antenna beams to provide downlink communication channels between the satellite and very small earth terminals. An example of such a communications satellite is disclosed in U.S. patent application Ser. No. 896,983 owned by the assignee of the present application. In the system disclosed in that application, a communication satellite interconnects large numbers of very small aperture earth terminals in a manner which maximizes satellite EIRP as well as the available bandwidth. The system employs highly directional, contiguous beams on the downlink which substantially increases the EIRP and allows multiple reuse of the assigned frequency spectrum. As a result, the number of communication channels that can be provided for point-to-point service is maximized. The high multi-carrier transmitter efficiency is achieved as a result of the dispersion of intermodulation products and the deleterious effects of rain on the downlink channels are easily overcome by the use of pooled transmitter power. The interconnection of the many users is achieved by a combination of a filter interconnection matrix and a highly directional addressable downlink beam.
The present beam-forming network is intended to overcome each of the deficiencies of the prior art and may be employed with a satellite communication system of the type discussed above.